This invention relates to a laser light source suitable for displacement and dispersion measuring interferometers, which can be used to measure displacements of high-performance stages, e.g., reticle and/or wafer stages, in a lithographic scanner or stepper systems and integrated circuit (IC) test equipment.
Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object. The light source for many displacement measuring interferometers is a single-wavelength, frequency-stabilized laser, see, e.g., xe2x80x9cRecent advances in displacement measuring interferometryxe2x80x9d by N. Bobroff, Measurement Science and Technology 4, 907-926 (1993). The accuracy of the displacement measurement varies directly with the wavelength stability of the light source.
In many applications, the measurement and reference beams have orthogonal polarizations and frequencies separated by a heterodyne, split-frequency. The split-frequency can be produced, e.g., by Zeeman splitting, by acousto-optical modulation, or by positioning a birefringent element internal to the laser. A polarizing beam splitter directs the measurement beam along a measurement path contacting a reflective measurement object, directs the references beam along a reference path, and thereafter recombines the beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that passes through a polarizer that mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a xe2x80x9cheterodynexe2x80x9d signal at the split frequency. When the measurement object is moving, e.g., by translating a reflective stage, the heterodyne signal is at a frequency equal to the split frequency plus a Doppler shift. The Doppler shift equals 2xcexdp, where xcexd is the relative velocity of the measurement and reference objects, xcex is the wavelength of the measurement and reference beams, and p is the number of passes to the reference and measurement objects. Changes in the optical path length to the measurement object correspond to changes in the phase of the measured interference signal, with a 2xcfx80 phase change substantially equal to an optical path length change nL of xcex/p, where n is the average refractive index of the medium through which the light beams travel, e.g., air or vacuum, and where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object.
For high performance applications such as IC manufacturing the quantity of interest is the geometrical length L and not the optical path length nL, which is what is A measured by the displacement measuring interferometer. In particular, changes in nL can be caused by changes in the refractive index n rather than by geometric changes in the relative position of the measurement object. For example, in lithography applications air turbulence, particularly in the region surrounding a moving wafer or reticle stage, can cause changes in n. Such changes need to be determined to obtain accurate geometric displacement measurements. If not corrected, the overlay performance and yield of a lithography tool used to manufacture ICs can be seriously limited. See, e.g., xe2x80x9cResidual errors in laser interferometry from air turbulence and non-linearity,xe2x80x9d by N. Bobroff, Appl. Opt. 26, 2676-2682 (1987).
Techniques based on dispersion interferometry have been used to compensate displacement measurements for air turbulence. In particular, interferometric displacement measurements are made at multiple optical wavelengths to determine the dispersion of the gas in the measurement path. The dispersion measurement can be used to convert an optical path length measured by a distance measuring interferometer into a geometric length. The conversion also requires knowledge of an intrinsic value for the refractivity of the gas. A suitable value is xcex93, which is the reciprocal dispersive power of the gas for the wavelengths used in the dispersion interferometry. In general, the sensitivity of the dispersion measurement to the consequences of air-turbulence correction increases as xcex93 decreases.
The invention features a displacement and dispersion measuring interferometry system having a Helium-Neon laser light source. The light source can be a Helium-Neon laser that includes an intracavity doubling crystal and an intracavity etalon to generate two harmonically related, single-frequency wavelengths at sufficient powers for interferometric dispersion measurements. Alternatively, the light source can be a single-mode Helium-Neon laser that directs a single-frequency input beam into a resonant external cavity enclosing a doubling crystal to generate two harmonically related, single-frequency wavelengths at sufficient powers for interferometric dispersion measurements. In addition to dispersion measurements, the inherent wavelength stability of the Helium-Neon source permits high-accuracy displacement measurements. Thus, the Helium-Neon laser light source is sufficient for the interferometry system to simultaneously measure displacement and dispersion, and correct the displacement measurement for air-turbulence using the dispersion measurement.
In general, in one aspect the invention features a Helium-Neon laser light source including: a Helium-Neon gain medium; a power source electrically coupled to the gain medium which during operation causes the gain medium to emit optical radiation at a first wavelength; a nonlinear optical crystal which during operation converts a portion of the optical radiation at the first wavelength into optical radiation at a second wavelength that is a harmonic of the first wavelength; an etalon; and at least two cavity mirrors enclosing the gain medium, the non-linear optical crystal, and the etalon to define a laser cavity, wherein during operation the etalon causes the cavity to lase at a single axial mode, and wherein at least one of the cavity mirrors couples the optical radiation at the first and second wavelengths into two harmonically related, single-frequency, output beams at the first and second wavelengths.
Embodiments of the laser light source can include any of the following features. A birefringent filter can be positioned within the cavity and oriented to select a particular Helium-Neon laser transition. The front and back faces of the crystal though which the optical radiation propagates can be parallel to one another to within 1 mrad. The at least two cavity mirrors can include two end mirrors and at least one fold mirror. The at least one fold mirror can have a coating that is less than 4% reflective at 3.39 microns.
Also, the laser light source can further include a detector and an intensity controller. During operation the detector measures an intensity of a portion of the output beam at the first wavelength and sends an intensity stabilization signal to the intensity controller indicative of the intensity of the output beam at the first wavelength. The intensity controller causes the power source-to adjust current flow through the gain medium based on the intensity stabilization signal.
Furthermore, the laser light source can include different embodiments for the Helium-Neon light source. For example, the Helium-Neon gain medium can include a vacuum tube filled with Helium and Neon gases, the tube having opposite ends with a Brewster window at one end and a bellows hermetically sealing the other end to one the cavity mirrors. Also, the Helium-Neon gain medium can include multiple vacuum tubes each filled with Helium and Neon gases and multiple fold mirrors folding the multiple tubes into the laser cavity. Furthermore, the Helium-Neon gain medium can include an enclosure of Helium and Neon gases, the enclosure having an elongate cross-section and being surrounded at opposite ends by mirrors that define multiple passes through the enclosure within the laser cavity.
In general, in another aspect, the invention features a Helium-Neon laser light source including: a single-mode Helium-Neon laser which during operation generates a single-frequency input beam at a first wavelength; a nonlinear optical crystal external to the laser which during operation converts a portion of the input beam at the first wavelength into optical radiation at a second wavelength that is a harmonic of the first wavelength; and a plurality of mirrors enclosing the nonlinear crystal to define a resonant external cavity, wherein one of the mirrors couples optical radiation at the first wavelength from the input beam into the external cavity and another one of the mirrors couples optical radiation at the first and second wavelengths out of the external cavity to produce two harmonically related, single-frequency, output beams at the first and second wavelengths.
Embodiments of either of the Helium-Neon laser light sources described above can include any of the following features. The two harmonically related, single-frequency, output beams can be coextensive. The intensity of each output beam can be greater than about 0.5 mW. The laser light sources can further include a transducer coupled to one of the cavity mirrors and a wavelength controller. During operation the wavelength controller causes the transducer to adjust the cavity length of the laser cavity or extra cavity, respectively, based on a wavelength stabilization signal derived from one of the output beams. For the laser cavity, for example, the wavelength stabilization signal can be generated by comparing the output frequency to the frequency produced by reference laser or a temperature-controlled Fabry-Perot cavity, or by analyzing the transmission of the output beam through a gas absorption cell having well-established absorption spectra. Alternatively, for the laser source having the external cavity, the cavity-length controller can cause the transducer to adjust the cavity length based on an error signal derived from input beam light not coupled into the external cavity.
Also, the laser light sources can her include first and second acousto-optical modulation systems positioned external to, the laser cavity or external cavity, respectively. During operation the first modulation system generates a frequency splitting between orthogonal polarization components of the output beam at the first wavelength and the second modulation system generates a frequency splitting between orthogonal polarization components of the output beam at the second wavelength.
Furthermore, the laser light sources can further include a heating element thermally coupled to the crystal and a temperature controller that causes the heating element to maintain a crystal temperature suitable for non-critical phase matching of the optical radiation at the first and second wavelengths. The non-linear optical crystal can have an optic axis oriented substantially perpendicular to the propagation direction of the optical radiation within the crystal. For example, the nonlinear optical crystal can be Rubidium Dihydrogen Phosphate (RDP). Alternatively, the nonlinear crystal can be oriented for critical phase matching of the optical radiation at the first and second wavelengths and can be, e.g., one of Lithium Triborate (LBO), Beta-Barium Borate (BBO), or Lithium Iodate (LiIO3).
In another aspect, the invention features an interferometry system including either of the Helium-Neon laser light sources described above and a dispersion interferometer, which during operation measures dispersion along a path to a measurement object using light derived from the two output beams.
In yet another aspect, the inventions features an interferometry system including either of the Helium-Neon laser light sources described above, an interferometer, and an optical analysis system. During operation the interferometer directs first and second measurement beams along a common path contacting a reflective measurement object, and combines the reflected first measurement beam with a first reference beam to form a first exit beam and the reflected second measurement beam with a second reference beam to form a second exit beam. The first measurement and reference beams are derived from the output beam from the laser light source having the first wavelength and the second measurement and reference beans are derived from the output beam from the laser light source having the second wavelength. The first and second exit beams are indicative of changes in the optical path length to the measurement object at the first and second wavelengths. During operation, the optical analysis system determines changes in the geometric path length to the measurement object based on the first and second exit beams.
In general, in yet another aspect, the invention features an interferometry system including: a Helium-Neon laser light source that generates two harmonically related, single-frequency output beams; and a dispersion interferometer which during operation measures dispersion along a path to a measurement object using light derived from the two output beams.
In general, in yet another aspect, the invention features an interferometry system including: a Helium-Neon laser light source that generates two harmonically related, single-frequency output beams; an interferometer; and an optical analysis system. During operation, the interferometer directs first and second measurement beams along a common path contacting a reflective measurement object, and combines the reflected first measurement beam with a first reference beam to form a first exit beam and the reflected second measurement beam with a second reference beam to form a second exit beam. The first measurement and reference beams are derived from the output beam from the laser light source having the first wavelength and the second measurement and reference beams are derived from the output beam from the laser light source having the second wavelength. The first and second exit beams are indicative of changes in the optical path length to the measurement object at the first and second wavelengths. During operation, the optical analysis system determines changes in the geometric path length to the measurement object based on the first and second exit beams.
In another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The system includes: a stage for supporting the wafer, an illumination system for imaging spatially patterned radiation onto the wafer, a positioning system for adjusting the position of the stage relative to the imaged radiation; and at least one of the interferometry systems described above for measuring the position of the stage.
In yet another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The system includes: a stage for supporting the wafer, and an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and at least one of the interferometry systems described above. During operation, the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position of the mask relative to the radiation from the source, the lens assembly images the spatially patterned radiation onto the wafer, and the interferometry system measures the position of the mask relative to the radiation from the source.
In yet another aspect, the invention features a lithography system for fabricating integrated circuits including first and second components and at least one of the interferometry systems described above. The first and second components are movable relative to each other. The first component includes the measurement object, and the interferometry system measures the position of the first component relative to the second component.
In yet another aspect, the invention features a lithography system for fabricating integrated circuits including first and second components, and at least one of the interferometry systems described above. The first and second components are movable relative to each other. The first component includes the measurement object, the reference beams contact the second component prior to forming the exit beams, and the interferometry system measures the relative position of the first and second components.
In yet another aspect, the invention features a beam writing system for use in fabricating a lithography mask. The system includes: a source providing a write beam to pattern a substrate; a stage supporting the substrate; a beam directing assembly for delivering the write beam to the substrate; a positioning system for positioning the stage and beam directing assembly relative one another, and at least one of the interferometry systems described above for measuring the position of the stage relative to the beam directing assembly.
In general, in another aspect, the invention features an interferometry method including: providing two harmonically related, single-frequency output beams from a Helium-Neon laser light source; and measuring dispersion along a path to a measurement object using light derived from the two output beams.
In general, in yet another aspect, the invention features an interferometry method including: providing two harmonically related, single-frequency output beams from a Helium-Neon laser light source; and interferometrically measuring changes in a geometric path length to a measurement object using light derived from the two output beams.
In another aspect, the invention features a lithography method for use in fabricating integrated circuits on a wafer. The lithography method includes: supporting the wafer on a moveable stage; imaging spatially patterned radiation onto the wafer, adjusting the position of the stage; and measuring the position of the stage using any of the interferometry methods described above.
In yet another aspect, the invention features a lithography method for use in the fabrication of integrated circuits. The lithography method includes: directing input radiation through a mask to produce spatially patterned radiation; positioning the mask relative to the input radiation; measuring the position of the mask relative to the input radiation using any of the interferometry methods described above, wherein one of a stage supporting the mask and a illumination system providing the input radiation includes the measurement object; and imaging the spatially patterned radiation onto a wafer.
In yet another aspect, the invention features a lithography method for fabricating integrated circuits on a wafer including: positioning a first component of a lithography system relative to a second component of a lithography system to expose the wafer to spatially patterned radiation; and measuring the position of the first component relative to the second component using any of the interferometry methods described above, wherein the first component includes the measurement object.
In yet another aspect, the invention features a beam writing method for use in fabricating a lithography mask. The method includes: directing a write beam to a substrate to pattern the substrate; positioning the substrate relative to the write beam; and measuring the position of the substrate relative to the write beam using any of the interferometry methods described above.
The invention has many advantages. The Helium-Neon laser light source generates two harmonically related, single-frequency wavelengths (632.8 nm and 316.4 nm) at powers sufficient for dispersion interferometry, e.g., output powers greater than 1 mW. The intracavity etalon causes the laser to operate in a single axial mode to thereby generate the single-frequency wavelengths. In addition because the Helium-Neon gain medium has a relatively narrow emission curve (about 1.5 GHz at 632.8 nm), the wavelength stability of the single-frequency wavelengths is excellent (about 3 parts in 106). Routine feedback control of the laser cavity length can further enhance the wavelength stability to better than 1 part in 109. Moreover, the Helium-Neon light source is compact, robust, and long-lived relative to many lasers with other gain media.
Furthermore, in embodiments for which the doubling crystal is non-critically phase matched by temperature tuning, there is no lateral walk-off between the fundamental and frequency-doubled beams. As a result, the laser light source output beams can have substantially circular transverse profiles, and the doubling crystal can be longer, thereby enhancing conversion efficiency. In addition, for either critically or non-critically phase-matched embodiments, an additional control system can modulate the current intensity to the Helium-Neon discharge tube based on the intensity of the fundamental output beam to independently stabilize the intensity of the fundamental output beam. Moreover, embodiments in which multiple Helium-Neon gas tubes are folded into the cavity or in which the cavity defines multiple passes within a Helium-Neon gas slab both increase the compactness of the laser and its intensity output. Also, some embodiments produce multiple pairs of harmonically related, single-frequency output beams thereby increasing the overall power generated by the light source.
The properties of the Helium-Neon laser light source can be exploited in a displacement and dispersion measuring interferometry system. The two high-stability, harmonically related, single-frequency wavelengths permit the interferometry system to simultaneously measure displacement and dispersion using the Helium-Neon laser light source as the only light source. The dispersion measurement can be used to correct the displacement measurement for air turbulence. In addition, the wavelengths provided by the laser (632.8 nm and 316.4 nm) are especially useful for the dispersion measurement because they correspond to a relatively low value for the reciprocal dispersive power xcex93 as compared with other wavelengths such as those from the fundamental and doubled output of a frequency-doubled Nd:YAG laser (1064 nm and 532 nm). In particular, xcex93 equals about 21.4 for the Helium-Neon laser wavelengths and xcex93 equals about 64.7 for Nd:YAG laser wavelengths. As mentioned above, the sensitivity of the dispersion measurement increases inversely with xcex93. Furthermore, the Helium-Neon wavelengths are in range (greater than about 300 nm) where suitable optical components and coatings can be fabricated inexpensively. Moreover, in embodiments for which the doubling crystal is non-critically phase-matched by temperature tuning, the symmetrical spatial profiles of the fundamental and frequency-doubled output beams reduce phase front errors in the interferometry measurements.
More generally, because the interferometry system can measure both dispersion and displacement using the Helium Neon laser light source as the only light source, the interferometry system is simple and compact. In particular, no additional light sources are necessary. Moreover, because the dispersion and displacement measurements are made using only two wavelengths, suitable coatings for the interferometer optics can be more easily obtained.
Nonetheless, in other embodiments, the interferometry systems can include an additional light source such as a separate Helium-Neon, Argon, or diode laser producing a single-frequency beam at only a single wavelength. In such cases, the Helium-Neon laser light source described above only provides light for interferometric dispersion measurements, and the additional light source provides light for interferometric displacement measurements.
Because the displacement and distance measuring interferometry systems provide high-accuracy position measurements corrected for air turbulence, they can be incorporated into lithography tools used to fabricate integrated circuits (ICs). The robustness and long life of the Helium-Neon source in the interferometry systems make the systems especially suitable for the demands of IC fabrication.
Other features, aspects, and advantages of the invention follow.